Communication devices and methods with hybrid beamforming

ABSTRACT

A communication device for RF-based communication with another communication device comprises digital beam-forming circuitry configured to perform digital beamforming based on digital beamforming information to obtain RF data streams, and analog beamforming circuitry configured to perform analog beamforming for the obtained RF data streams. The analog beamforming circuitry is configured to perform analog beamforming training with the other communication device enabling the other communication device to compute the digital beamforming information corresponding to one or more combinations of analog beams used in said analog beamforming training. The digital beamforming circuitry is configured to receive the computed digital beamforming information and to use it for performing the digital beamforming.

BACKGROUND Field of the Disclosure

The present disclosure relates to different communication devices, suchas a mobile station and an access point, which are configured forRF-based communication with each other. The present disclosure furtherrelates to corresponding communication methods.

Description of the Related Art

Communication systems in the 60 GHz frequency range suffer from strongfree space path loss, which increases with frequency. For instance, a 60GHz communication system has roughly 22 dB higher attenuation comparedto a communication system operating at 5 GHz. To overcome the increasedpath loss, 60 GHz or any mm-wave communication system employsbeamforming, i.e. transmitter and/or receiver feature a steerablephased-array antenna (PAA), which can form directive beams towards theother communication device. Such beams have typically a high directivityand are spatially very narrow. The directivity in main directionincreases with the number of antenna elements per PAA. In contrast, thehalf-power beam width (HPBW) defines the spatial width of a patterndecreases with increasing number of antennas. Thus, the more antennasper PAA, the higher the directivity and the smaller the HPBW. In orderto make use of the PAA directivity for communications, beam alignment iscrucial and of high importance for mm-wave communication systems and RFcommunication systems, methods and devices in general.

The goal of transmit beamforming techniques is to allow MIMOtransmitters to simultaneously send multiple streams to one or morereceivers, based on the channel states, such that an adequate receptioncan be achieved. As compared to the sub 6 GHz, mm-wave channels posemany additional challenges on the design of transmit beam-formers. Dueto the very high frequency, mm-wave channels suffer from strong pathloss, which can only be combatted by exploiting antenna arrays ofmultiple elements. However, the use of many antenna elements can haveprohibitive complexity both due to hardware constraint as well aschannel estimation burden. Therefore, hybrid antenna architectures whereonly a limited number of RF chains are employed to steer antenna arraysof multiple elements are more useful.

Single carrier (SC) transmission has been chosen by the developingmm-wave standards IEEE 802.11ad and IEEE 802.11ay as the mandatorymodulation mode, due to the reduced complexity. However, since largechunks of spectrum are available and envisioned to be used in mm-wavechannels, the channels will experience a certain frequency selectivityand equalization techniques are required. Frequency domain equalization(FDE) is a common equalization technique to reduce inter-symbolinterference.

Thus, there is a need for transmit beamforming schemes which are moreappropriate for the wideband SC mm-wave, without needing overlycomplicated filters at the transmitter or excessive FFT/IFFT blocks andtaking into account the equalization scheme at the receiver.

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventor(s), to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentdisclosure.

SUMMARY

It is an object to provide communication devices and correspondingcommunication methods enabling wideband SC mm-wave communication whichdo not require much additional or even complicated hard- or software,e.g. complex filters or FFT and IFFT blocks at the transmitter and whichcan be better adapted to the SC receiver structures

According to an aspect there is provided a (first) communication device(also called initiator or transmitter), e.g. an access point,comprising:

digital beamforming circuitry configured to perform digital beamformingbased on digital beamforming information to obtain RF data streams, and

analog beamforming circuitry configured to perform analog beamformingfor the obtained RF data streams, and

wherein said analog beamforming circuitry is configured to performanalog beamforming training with the other communication device enablingthe other communication device to compute the digital beamforminginformation for combinations of analog beams used in said analogbeamforming training, and

wherein said digital beamforming circuitry is configured to receive thecomputed digital beamforming information and to use it for performingthe digital beamforming.

According to a further aspect there is provided a (second) communicationdevice (also called responder or receiver), e.g. a station, comprising:

analog beamforming circuitry configured to perform analog beamformingtraining with the other communication device for determining analogbeams for use in receiving said RF data streams, and

digital beamforming computation circuitry configured to compute digitalbeamforming information based on a predetermined metric computed forcombinations of analog beams used in said analog beamforming trainingand to transmit the computed digital beamforming information to theother communication device enabling the other communication device toperform digital beamforming based on the digital beamforminginformation.

According to a further aspect there is provided a (third) communicationdevice (which is an alternative to the first communication device)forRF-based communication with another communication device, comprising

digital beamforming circuitry configured to perform digital beamformingbased on digital beamforming information to obtain RF data streams, and

analog beamforming circuitry configured to perform analog beamformingfor the obtained RF data streams,

wherein said analog beamforming circuitry is configured to performanalog beamforming training with the other communication device enablingthe communication device to compute the digital beamforming informationcorresponding to one or more combinations of analog beams used in saidanalog beamforming training, and

wherein said digital beamforming circuitry is configured to use thecomputed digital beamforming information for performing the digitalbeamforming.

According to still further aspects a computer program comprising programmeans for causing a computer to carry out the steps of the methoddisclosed herein, when said computer program is carried out on acomputer, as well as a non-transitory computer-readable recording mediumthat stores therein a computer program product, which, when executed bya processor, causes the method disclosed herein to be performed areprovided.

Embodiments are defined in the dependent claims. It shall be understoodthat the disclosed communication methods, the disclosed computer programand the disclosed computer-readable recording medium have similar and/oridentical further embodiments as the claimed communication devices andas defined in the dependent claims and/or disclosed herein.

One of the aspects of the disclosure is to provide low complexitydigital beamforming, in particular precoding, for single user (SU) MIMOmm-wave single carrier communications and feedback schemes to allow theapplication of the scheme. The beamforming information for use in thedigital beamforming at the transmitter is computed at a receiver,particularly based on channel information in the frequency domain, andis preferably translated into a wideband precoder to be used by thetransmitter. The obtained solution is then fed back to the transmitter,for which purpose required signaling is disclosed in embodiments. Thedisclosed solution leverages on the one hand the low complexity of thesingle carrier transmission and on the other hand the advantages ofequalization techniques e.g., frequency domain equalization at thereceiver to combat inter-symbol interference.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 shows a schematic diagram of a communication system includingfirst and second communication devices according to the presentdisclosure;

FIG. 2 shows a more detailed schematic diagram of a communication systemincluding a first communication device and a second communication deviceaccording to the present disclosure;

FIG. 3 shows a diagram illustrating a general embodiment of theoperations of communication methods according to the present disclosure;

FIG. 4 shows a diagram illustrating a first exemplary embodiment of theoperations of communication methods according to the present disclosure;

FIG. 5 shows a diagram illustrating a second exemplary embodiment of theoperations of communication methods according to the present disclosure;

FIG. 6 shows a diagram illustrating the operations of digital trainingafter RTS/CTS exchange with acknowledgement;

FIG. 7 shows a diagram illustrating the operations of digital trainingafter RTS/CTS exchange without acknowledgement; and

FIG. 8 shows a diagram illustrating the operations of digitalbeamforming without RTS/CTS exchange directly after analog BF trainingand with acknowledgement.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views, FIG. 1shows a schematic diagram of a communication system 100 including firstcommunication devices 1, 3 (stations STA1, STA2) and a secondcommunication device 2 (an access point AP). The communication devicesare generally configured to carry out RF-based communication with eachother.

Each of said communication devices 1, 3 serving as receiver, generallycomprises digital beamforming circuitry 10, 30 (also called digitalbeamformer or digital beamforming unit herein) configured to performdigital beamforming based on digital beamforming information to obtainRF data streams, analog beamforming circuitry 11, 31 (also called analogbeamformer or analog beamforming unit herein) configured to performanalog beamforming for the obtained RF data streams, and antennacircuitry 12, 32 configured to transmit the obtained RF data streamsusing the analog beams formed by the analog beamforming circuitry. Theanalog beamforming circuitry 11, 31 is hereby configured to performanalog beamforming training with the other communication device enablingthe other communication device to compute the digital beamforminginformation based on a predetermined metric computed for combinations ofanalog beams used in said analog beamforming training. The digitalbeamforming circuitry 10, 30 is configured to receive the computeddigital beamforming information and to use it for performing the digitalbeamforming.

Said communication device 2, serving as transmitter, generally comprisesantenna circuitry 22 configured to receive RF data streams, analogbeamforming circuitry 20 configured to perform analog beamformingtraining with the other communication device for determining analogbeams for use by the antenna circuitry to receive said RF data streams,and digital beamforming computation circuitry 21 configured to computedigital beamforming information based on a predetermined metric computedfor combinations of analog beams used in said analog beamformingtraining and to transmit the computed digital beamforming information tothe other communication device enabling the other communication deviceto perform digital beamforming based on the digital beamforminginformation.

The disclosed communication devices enable transmitting beamformingschemes which are more appropriate for the wideband SC mm-wave.Precoding matrices may be computed and fed back based on the assumptionthat the precoding is done per subcarrier or per group of subcarriers.This does, however, not hold in the case of SC mm-wave systems, whichare of interest here. Further, known mm-wave systems do not containstructures to allow for the feedback and application of digitalbeamformers.

An embodiment of this disclosure refers to a wideband hybrid beamformingsolution for SU MIMO, which can be easily applied on conventional SCmm-wave communication links. Aspects of the present disclosure include adecoupled training, in which in a first stage analog beamformingtraining is performed to find the phase shifters configuration that bestexpose the channel and promises the largest capacity and a second stagein which the digital beamformers are found at the receivers based onfrequency domain channel state information. A further aspect refers to amethod to obtain an optimal transmit precoder, which is constant overfrequency, based on the channel information in frequency domainavailable at the receiver. Further aspects refer to heuristic solutionswhich can be used for easier practical implementations and feedbackschemes, e.g. mapped to the IEEE 802.11ay signal flows and framestructure.

FIG. 2 depicts a detailed schematic diagram of an embodiment of acommunication system 200 including a first communication device 4 (e.g.an access point, AP, serving as transmitter) and a second communicationdevice 5 (e.g. a station STA serving as receiver) according to thepresent disclosure.

The first communication device 4 comprises a data mapping unit 40, aguard interval insertion unit 41, a digital beamforming unit 42, severalRF chains 431, 432 and, per RF chain, an analog beamforming unit 441,442, and antenna unit 45. RF signals are transmitted over the channel 6to the second communication device 5, which comprises an antenna unit50, several RF chains 521, 522, an analog beamforming unit 511, 512 perRF chain, a channel estimation unit 53, a digital beamformingcomputation unit 54 an equalization unit 55 and a detection unit 56. Theoperation of the communication devices 4 and 5 will be explained in moredetail below. The second communication unit 5 may further comprise,before the channel estimation, a guard interval removing unit and a DFTunit and, after the equalization, an IFFT unit (not shown).

Assume that N_(s) streams are sent, which for ease of exposition areconsidered composed of just one block of M symbols and N_(c)−M guardinterval symbols i.e.,

s _(i) =[s _(i) ⁽¹⁾ , s _(i) ⁽²⁾ , . . . , s _(i) ^((M)) , s _(i)^((M+1)) , . . . , s _(i) ^((N) ^(c) ⁾].

For ease of exposition consider_(N) the transmitted symbol vector as=[s₁⁽¹⁾, . . . , s_(M) ⁽¹⁾, s₁ ⁽²⁾, . . . , s_(M) ⁽²⁾, . . . , s₁ ^((N) ^(c)⁾, . . . , s_(M) ^((N) ^(c) ⁾]. The transmitted signal is x=P_(A)P_(D)s,where P_(D)and P_(A) denote the transmit digital and analog beamformingmatrices, respectively. More clearly, the analog matrix is representedas block diagonal, with all blocks equal, to model that the analogbeamformer does not change during the transmission of a symbol block. Onthe other hand P_(D) can model a filter with number of taps no largerthan the guard interval length, thus it is chosen as a block circulantmatrix. The received symbol after GI removal is=W_(A)^(H)HP_(A)P_(D)s+n, where H=blkcirc([H⁽¹⁾, . . . , H^((L)), 0⁽¹⁾, . . ., 0^(N) ^(c) ]) and W_(A) is the receive analog beamforming matrix.W_(A) ^(H) is block diagonal matrix denoting the analog receivebeamforming matrix. The effective channel which is seen after the analogtransmit beamforming and receive beamforming can be written asH_(eff)=blkcirc([W_(BA) ^(H)H⁽¹⁾P_(BA), . . . , W_(BA) ^(H)H^((L) ^(eff)⁾P_(BA), 0⁽¹⁾, . . . , 0^(N) ^(c) ]), where L_eff represents the numberof effective taps remaining after applying the analog beamformingoperations P _(BA) and W_(BA) denote repeating diagonal blocks of theprecoding matrix and combining matrix respectively.

For a given precoding matrix, the receive strategy that minimizes theMMSE is

W _(D)=(I+P _(D) ^(H) H _(f,eff) ^(H) H _(f,eff) P _(BD)))⁻¹ P _(D) H_(f,eff),

where H_(f,eff) ^(H)H_(f,eff) is a block diagonal matrix and each blockk, corresponds to the channel matrix at subcarrier k. By imposing thatthe same digital transmit beamforming matrix is used for allsubcarriers, is equivalent to assuming P_(D)=P_(BD)⊗I

W _(D)=(I+(P _(BD) ^(H) ⊗I)H _(f,eff) ^(H) H _(f,eff)(P _(BD) ⊗I))⁻¹(P_(BD) ^(H) ⊗I)H _(f,eff) ^(H).

The digital transmit matrix which both maximizes the rate and minimizesthe MMSE can be found by solving the following optimization problem (1)

${\max\limits_{P_{BD}}{\sum\limits_{k = 1}^{N_{c}}{{{logdet}\left( {I + {P_{BD}H_{fk}H_{fk}^{H}P_{BD}^{H}}} \right)}{s.{to}}\mspace{14mu} {Tr}\left\{ {P_{BD}P_{BD}^{H}} \right\}}}} \leq {P.}$

Differently than conventional solutions, the mutual informationmaximization does not directly result from the SVD decomposition due tothe coupling of P_(BD) to all frequency domain channels. However, themutual information maximization and the MMSE minimization are equivalentto a convex optimization problem, which can be solved within requiredprecision by e.g. interior point solvers. To expose the convexstructure, it can be noticed that problem (1) can be equivalentlyreformulated as an optimization problem in the positive semidefinitematrix Q_(BD) as

max_(Q) _(BD) Σ_(k=1) ^(N) ^(c) logdet(I+Q _(BD) H _(fk) H _(fk) ^(H))s. to Tr{Q _(BD) }≤P, Q _(BD)≥0

Problem (2) represents a maximization of a concave function over aconvex set, therefore is within the category of semi-definite problems,for which algorithms are known.

To verify that the solution is optimal various criteria can be used,e.g., based on KKT (Karush-Kuhn-Tucker) conditions or by imposing aduality gap smaller than a threshold. Suboptimal solutions can also befound based on projection methods. For example if the optimal solutionfor an unconstrained transmit beamforming matrix can be computed basedon the channel information in frequency domain, e.g.

P _(BD)=argmin_(P) _(BD) d(P* _(D),(P _(BD) ⊗I)),

where P*_(D) is the optimum precoding matrix defined for carriercooperative or subcarrier based block diagonal and. d(*,*) represents adistance measure (can be norm 2 or Frobenius norm of the differencebetween the arguments or chordal distance). In both cases closed formsexist and are based on the CIRs (Channel Impulse Response) in thefrequency domain. Another practical method to obtain the precodingmatrix is to consider an average of the channel covariance matrix infrequency domain i.e.,

$R = {\frac{1}{N_{c}}{\sum_{k}{{JH}_{fk}H_{fk}^{H}}}}$

and compute P_(BD) as the eigenvector of this covariance matrix.

FIG. 3 shows a diagram illustrating an embodiment of current operationsof communication methods according to the present disclosure. Thismethod has been adopted to allow for the beamforming training. Withinthe MIMO Setup Frame various parameters for the future training stageare negotiated such as number of BRP frames to be sent, TX sectors to betested, etc. During I-SMT (initiator SU MIMO training) the initiator(e.g. an AP, second communication device) sends BRP frames to train itsanalog beams. Similarly during R-SMT (responder SU MIMO training), theresponder (e.g. a STA, first communication device) trains analog beamsvia BRP frames. Finally feedback information containing obtained valuesfor the combinations of analog beams and channel measurements are fedback during the SU MIMO Feedback Phase. In the disclosed scheme, thesestages can e.g. be used to find the most promising analog matricesP_(BA) and W_(BA), for P_(BD)=I, i.e., when no digital beamforming isapplied at the transmitter.

To allow for digital beamforming schemes to be performed additions aremade in embodiments of the disclosed devices and methods to the MIMOSetup frame and the MIMO feedback. Since computing the digitalbeamformers is a more tedious task, it is proposed to have the solutioncomputed only for the best combination(s) of analog beams. Asking forthe computation of the digital beamformers can be done within the MIMOFeedback frames and sent as proposed elements in further MIMO feedbackframes or as proposed beamforming frames. One additional transmissioncan be performed at the end of the training as a BF (beamforming) poll,to allow the test of the newly formed digital beam. Further, a DigitalBF SMT can be conceived, in which the AP transmits with both digital andanalog beamforming.

Beamforming can be performed either by a STA or by an AP. If performedby a STA, then this can compute the precoding matrix that the AP shoulduse and feedback. If computed by an AP, this needs to feed it back tothe STA, such that this can adapt its receive beamforming scheme andequalizer filters. In any case, signaling is included for variouspurposes:

i) to indicate that digital beamforming training will be performed,

ii) to indicate which of the communication devices is performing thebeamforming matrix computation;

iii) to transmit the digital beamforming matrices (generally calledbeamforming information herein);

iv) optionally to indicate the format and required parameters of thedigital precoders (should only one format be allowed, then this can bepredefined e.g. in the communication devices and/or in a standard; and

v) to indicate the capabilities of the devices to compute or applydigital precoders.

Several embodiments for the various kinds of signaling are furtherdiscussed below. These fall into two main categories:

a) All required signaling (i-iv) is performed with BRP (Beam RefinementPhase). Advantages are flexibility as well as possibility to request thedigital beamforming only on particular BRP packets, or to requiredigital beamforming computation within a tracking process. A furtheradvantage is that STAs can easily recommend application of digital BFbased on the quality of the channels at their receivers or can requestdigital beamforming training together with RX beamforming training orsimply digital beamforming training to be applied for the current analogbeam combinations.

b) Required signaling is introduced in various parts in the MIMOtraining setup and MIMO feedbacks, according to the signal flow depictedin FIG. 3.

A first embodiment for implementing signaling uses a modification of theBRP frames. In this case the MIMO feedback is performed with the sametype of frames, i.e., BRP frames, as the rest of the beamforming process(e.g., analog beam-training setup, analog beam feedback, channelmeasurement). The structure of the BRP frame is shown in Table 1 below,wherein in the blocks printed in bold letters some smallmodifications/additions are made.

TABLE 1 BRP frame Action field format Order Information 1 Category 2Unprotected DMG Action 3 Dialog Token 4 BRP Request field 5 DMGRefinement element 6 Zero or more channel measurement feedback 7 EDMGBRP Request Element 8 Zero or more EDMG channel measurement Feedback 9Zero or more Beamforming Feedback Elements

The required changes for each of the blocks may be provided asillustrated in the following non-limiting exemplary embodiments:

i) Indication (also called “digital beamforming indicator” herein) thatdigital BF should be performed within the training: This can be includedin the EDMG (enhanced directional multi-gigabit) BRP Request Element asfollows:

Element Element Element L- L-TX- Tx EDMG EDMG EDMG Digital Reserved IDLength Extension RX RX Sector TRN TRN TRN BF ID Unit P Unit M Unit NRequest

If the Digital BF Request is present then digital beamformingcomputation is performed on the next BRP packet for the analog beamswith which the BRP packet is sent. The indication regarding the STAwhich is computing these matrices is inferred from the feedback typepresent in the Refinement Element, as shown further below.

ii) Indication (also called “decision indicator” herein) whichcommunication device is performing the precoding computation: Thepresence or the request for channel measurement feedback can beindicated in the DMG Refinement Element depicted below. Similarly, thepresence and request for digital beamforming feedback should also beindicated in this element as follows.

B27 B0 B19 B20 B21 B28 B29 B7 B8 B17 B18 TX- TXSS- B26 BS- B33 ElementB15 B16 TX-train- RX-train- TRN- FBCK- BS- FBCK FBCK- ID LengthInitiator response response OK REQ FBCK Antenna ID REQ Bits: 8 8 1 1 1 11 6 2 5 B56 B60 B61 B66 B34 B59 BS- B64 B65 EDMG B51 B52 B53 B54 BS-FBCK Number of EDMG Channel FBCK- MID Capability B55 FBCK AntennaMeasurements Extension Measurement TYPE Extension Request Reserved MSBID MSB MSB Flag Present Bits: 18 1 1 2 4 1 4 1 1 B67 Short SSW FBCK-FBCK- Packet Used REQ-Ext TYPE-Ext Reserved Bits: 1 2

A value of 1 in the FBCK-Req-Ext implies that requested feedbackcontains the digital beamforming matrix. If Digital BF Request=1 in theDMG Request Element and FBCK-Req-Ext=1 in the DMG Refinement element,then the computation is being performed by the STA receiving the BRPpacket, for the effective link corresponding to the analog beamformersutilized for receiving the BRP packet. Many implementations may prefer acomputation with one beamforming matrix for the whole frequency band.However, if FIR precoding filters with multiple taps are desired, Ntapscan be re-interpreted as the number of non-null elements of the FIRprecoding filter Ntaps_b, as generally there is no need for both channelfeedback information and digital beamforming matrices.

If FBCK-Type-Ext is 1, beamforming feedback information (also called“precoding information” herein) is present within the packet. Further ifthis bit is set, however the Channel Measurement Present is 0 and Ntapsis non-null within the FBCK-Type the beamforming feedback containsmultiple taps for an FIR filter corresponding to the chosen digitaltransmit beamformer.

iii) Feedback Content (also called “beamforming information” herein):The beamforming matrix can be contained in a newly defined field in theChannel Measurement Feedback Element, as shown in Table 2:

TABLE 2 EDMG Channel Measurement Feedback element format Field SizeMeaning Element ID 8 bits Defined in 9.4.2.1 Length 8 bits Defined in9.4.2.1 Element ID Extension 8 bits Defined in 9.4.2.1 EDMG Sector IDSector ID₁/CDOWN₁ 11 bits Order TX Antenna ID₁ 3 bits RX Antenna ID₁ 3bits Sector ID₂/CDOWN₂ 11 bits TX Antenna ID₂ 3 bits RX Antenna ID₂ 3bits . . . . . . Sector ID-_(Nmeas)/ 11 bits CDOWN_(Nmeas) TX AntennaID_(Nmeas) 3 bits RX Antenna ID_(Nmeas) 3 bits Beam Tracking TX SectorCombination 1 11 bits Contains the AWV for TX DMG antenna 1 Feedback AWV1 TX Sector Combination 1 11 bits Contains the AWV for TX DMG antenna 2AWV 2<<< . . . . . . TX Sector Combination 1 11 bits Contains the AWVfor TX DMG antenna AWV N_(TX) N_(TX) TX Sector Combination 2 11 bitsContains the AWV for TX DMG antenna 1 AWV 1 TX Sector Combination 2 11bits Contains the AWV for TX DMG antenna 2 AWV 2 . . . . . . TX SectorCombination 2 11 bits Contains the AWV for TX DMG antenna AWV N_(TX)N_(TX) . . . . . . . . . TX Sector Combination 11 bits Contains the AWVfor TX DMG antenna 1 N_(meas) AWV 1 TX Sector Combination 11 bitsContains the AWV for TX DMG antenna 2 N_(meas) AWV 2 . . . . . . TXSector Combination 11 bits Co<ntains the AWV for TX DMG antenna N_(meas)AWV N_(TX) N_(TX) Digital TX BF Digital TX Beamforming Ntaps_b × Definedin table 3 Feedback Matrix - Nb1 × Nt × Nr + Ndelays × Nb2 × Nt × Nr

By default, the Transmit Beamforming matrix contains information on onedigital precoding matrix, (i.e., Ntaps_b=1) displayed in compressed oruncompressed form. An example for uncompressed form is given in Table 3.This matrix is computed at the receiver, based on its equalizer and thisoperation can be performed with one of the methods shown above. For theexamples above TX Digital BF 1 is the element of the first row and thefirst column of P_BD, written as real part followed by imaginary part.Nb1 and Nb2 represent the number of bits for quantization of each of thereal part and the imaginary part of each beamforming tap and delay,respectively. In Table 3 N_CB denotes the number of bonded channel andT_c the chip rate.

TABLE 3 Digital TX BF Feedback Field Digital TX BF Feed- TX Digital BF 1Ntaps_b × Nb1 TX BF coefficient(s) for stream back 1 to TX antenna 1 TXDigital BF 2 Ntaps_b × Nb1 TX BF coefficient(s) for stream 2 to TXantenna 1 . . . . . . . . . TX Digital BF Ns × Ntaps_b × Nb1 TX BFcoefficient(s) for stream Nt Ns to TX antenna Nt Digital TX BF FIR TXDigital BF 1 Ntaps_b × Nb2 Delays in units of T_c/N_CB Delays Delaysbetween each two consecutive taps for stream 1 to antenna 1 . . . . . .. . . TX Digital BF Ns × Ntaps_b × Nb2 Delays in units of T_(c)/N_(CB)Nt Delays between each two consecutive taps for stream Ns to antenna Nt

The Digital TX BF can be included as a field in the EDMG ChannelMeasurement Element or can be defined as a standalone feedback elementAlternatively, the precoding information can be signaled in compressedform by means of e.g., Givens rotation or codebook entry indices.

For ease of implementation and better similarity to the OFDM typefeedback, also for the SC case the power of the beamformers (e.g.beamforming weights) for the specific streams can be separated from aunitary part. This means that the P_(BD) matrices obtained at areceiver, with one of the methods disclosed herein can be decomposed as

$\begin{matrix}{P_{BD} = {\begin{bmatrix}{SNR}_{1} & \cdots & 0 \\\vdots & \ddots & \vdots \\0 & \cdots & {SNR}_{N_{SS}}\end{bmatrix}\begin{bmatrix}u_{11} & \cdots & u_{N_{t},1} \\\vdots & \ddots & \vdots \\u_{1,N_{SS}} & \cdots & u_{N_{t},N_{SS}}\end{bmatrix}}} & (1)\end{matrix}$

with u_(i)=[u_(1,i), . . . u_(N) _(t) _(,i)]^(T), the beamforming vectorcorresponding to the stream I being unit norm. In this case, the TX BFcoefficient of stream j to antenna i is represented by the real part ofu_(ij), followed by the imaginary part as shown in Table 3′ of the formbelow. In particular, an alternative to the SC BF feedback presented inTable 3 is shown in Table 3′. The SNR blocks can be provided within thiselement or separately contained in a channel measurement element.

TABLE 3′ Digital TX BF Feedback Field (alternative) SNR SNR 1 8 SNR forstream 1 SNR 2 8 SNR for stream 2 SNR Nss 8 SNR for stream Nss DigitalTX BF TX Digital BF 1 16 TX BF coefficient(s) Feed-back for stream 1 toTX antenna 1 TX Digital BF 2 16 TX BF coefficient(s) for stream 1 to TXantenna 2 . . . . . . . . . TX Digital BF 16 TX BF coefficient(s) Nss ×Nt for stream Ns to TX antenna Nt

To obtain the digital feedback, a digital BF initiator may transmit withan analog beamforming combination which has been chosen as part of thepreceding analog beamforming training. It may use all TX antennascorresponding to the combination and attach to BRP frames enough TRN(training) fields to allow precise channel estimation at all the receiveantennas of the responder (with desired tap resolution). Further, ifnecessary, the TRN fields may be multiplied with an orthonormal matrix Tto allow a good estimation. For instance, the T matrix may correspond toa matrix with cyclic shift delays on the main diagonal.

After receiving a feedback in the form specified in Table 3′,u_(i,j)=Re{u_(i,j)}+jIm{u_(i,j)} is reconstructed and then P_(BD) ofdimension N_(tx)xN_(SS) is computed by the initiator, based on equation(1). The initiator will then use this information, i.e. P_(BD), to buildthe digital beamforming matrix used by itself in the subsequenttransmission P_(BD) ^(TX). The steering matrix that may be used by thetransmitter is in this case P_(BD) ^(TX)=P_(BD) (if no T is applied) ora transformed matrix P_(BD) ^(TX)=T·P_(BD), where T is an orthonormalmatrix, which has been used during the transmission of the TRN fieldsused for the channel estimation of H_(eff)=W_(A)HP_(A), based on whichthe design of P_(BD) has been done.

iv) Format and parameter setup (also called “precoding indicator”herein; sometimes also called digital beamforming control information orprecoding control information or precoding parameter information): Sincecurrently the number of bits for quantization used for the channelmeasurement feedback is fixed and a format for the digital precoders isproposed that follows the format of the former, it is expected thatthese numbers will also be fixed for the latter. However, these numberscan be alternatively signaled and this is shown further hereinafter.Optionally if the transmitter has capabilities of adapting precodingcoefficients by means of FIR filters, one precoding matrix can bedefined per tap and delay according to the filter capabilities of thetransmitter. The most precise form however is to further include aDigital BF Parameter Setup in the BRP Frame action as shown in Table 1,after the EDGM BRP Setup. Within the BF Feedback parameters setup thefollowing may be present: format of the beamforming matrices (compressedor uncompressed), whether the beamforming matrices are represented intime domain (preferable for SC) or frequency domain (preferable forOFDM), Number of streams, Number of bits for quantization (Nb1, Nb2), anindication of whether digital beamforming is recommended and/or how muchthe performance increases based on the computed value of a prede-finedperformance metric.

In an alternative embodiment to reusing the BRP frame a new frame isdesigned for the digital beamforming. A possible solution is shown inTable 4:

TABLE 4 Digital Beamforming Frame 1 Category 2 Unprotected DMG 3 DialogToken 4 BF Feedback Parameters Setup Element 5 BF Feedback Element

The BF Feedback parameters setup can be defined as proposed above,whereas for SC the BF Feedback Element contains after the Element ID andLength, the Digital BF Feedback Field as defined in Table 3.

A second embodiment for implementing signaling uses the MIMO Setup andMIMO Feedback structures as shown in FIG. 3. Similar to the descriptionof the first embodiment using a modification of the BRP frame,particularly regarding beamforming feedback in the channel measurementfeedback or as a separate frame, in this case the MIMO Training Controlfield is the one carrying FBType information, which contains informationif Digital TX Beamforming is present and optionally informationregarding the Ntaps and non-null Ndelays to indicate the length of thetransmit filters and, respectively that Ndelay is present. Furthermore,in the FBType of the MIMO Control field, information about the number ofbits for quantization Nb1, Nb2 can be present.

i) and iii) Indication that Digital Beamforming will be performed duringthe training and which communication device is performing the trainingwould be in this case in the MIMO Setup:

TABLE 5 MIMO Setup frame Action field format Order Information 1Category 2 Unprotected DMG Action 3 Dialog Token 4 EDMG BRP Requestfield 5 MIMO Setup Control element 6 One or more EDMG MIMO BRPConfiguration elements 7 Zero or more TX BF Config Elements

The MIMO Setup Control Element, shown in Table 6 contains among otherinformation about communication device is the decision maker for aparticular link and the type of feedback requested (in aMIMO-REQ-Field). In the case of digital beamforming this fieldadditionally contains whether digital beamforming is considered in thetraining or not, e.g. signaled within a DigitalBF field. Furthermore theMIMO-Req-Field uses one field to request digital bf feedback. Thisserves in this case as decision indicator as follows: If DigitalBF=1 andDigitalBF_FBCK=1, the STA to which the packet is intended will computethe precoding information. If DigitalBF=1 and DigitalBF_FBCK=0 signalsthat the station sending the request will compute the precoding matrixfor the particular link based on the channel measurement feedback.

The EDMG MIMO BRP Configuration Element contains the antenna weightvectors that are to be used.

TABLE 6 MIMO Setup Control Element Field Name Description ElementSpecific Information ID, Length, Extension Initiator Indicates if senderof the packet is the initiator of the bf training L-Tx-Rx Number of BRPpackets Link Type Initiator or responder link Decision Maker Indicatesif the sender of the packet is the decision maker for the link DigitalBF Indicates that digital BF computation will be performed MIMO-FBCK-ReqType of Feedback Requested - updated to include Digital BF Request asshown below MU Specific Info Number of Groups/Users per Group

MIMO FBCK Req with implicit information of the digital BF format maylook as follows:

Fields SNR Channel Ntaps EDMG TX Digital BF Request Measurement Ordercombinations FBCK Request Indication Request

MIMO FBCK Req with explicit information of the digital BF format maylook as follows:

Fields SNR Channel Ntaps EDMG TX Digital Digital Request MeasurementOrder combinations BF BF Request Indication Request Format

iii) The Digital Beamforming Feedback and Parameters: When theDigitalBF_FBCK Field is 1, the Ntaps field can be re-interpreted as thenumber of taps required for BF feedback, i.e. Ntaps=Ntaps_b. Anotheroption is to include within the MIMO-Req-Field (within the MIMO TrainingControl defined in Table 6) specific information about the format of therequired digital feedback: i.e., Ntaps_b and number of bits forquantization. Within the MIMO training control element, the number ofstreams that can be formed for the particular channel states can beadditionally fed back.

TABLE 7 MIMO Feedback frame Action field format Order Information 1Category 2 Unprotected DMG Action 3 Dialog Token 4 MIMO Training Controlelement 5 One or more EDMG Channel Measurement Feedback elements 6 Zeroor more Beamforming Feedback Elements

The MIMO Feedback frame may additionally contain an indication ofwhether the receiver recommends the use of digital beamforming for aparticular link, according to the channel states. Thus e.g., a respondermay signal that for particular channel realization, the application ofthe digital beamformer does not produce a significant increase inperformance. In this case it may not include the precoding matrixinformation in the feedback and due to the recommendation indication theinitiator also does not re-attempt digital beamforming training for theparticular channel. The performance metric based on which therecommendation is set can be defined as capacity gain, average SINRgain, MMSE, average MSE where gain is with digital beamforming appliedrelative to when only analog beamforming is applied.

In the following two exemplary embodiments for the communication betweentwo communication devices 4 and 5 will be described.

In a first exemplary embodiment illustrated in FIG. 4 the initiatorwants to train the analog and digital beamformers for the initiator toresponder link. In this example it is assumed that the decision makerfor the initiator link is the responder and that the initiator relies onthe responder's receiver to compute and feedback the digital transmitbeamformers that the former will use for transmission to the responder.

In a first step, similar to current training flows, the initiatorsignals within the MIMO Setup the analog beams combinations with whichit will transmit sequentially during a second step. In this case it willset the decision maker to 0 to indicate to the responder that the latteris the decision maker. Enabling digital beamforming in addition toanalog training can be done with the proposed structures by setting theDigital BF within the MIMO Control Setup to 1. Furthermore it sets theDigital BF-FBCK-Req within the MIMO FBCK Req to 1. This signals that itis the responder which will need to do the digital beamformingcomputations. In MIMO FBCK Request it may indicate the required numberof taps that the precoding filter may have e.g., Ntaps_b=1, and numberof bits for quantization.

In a second step, in each BRP packet the initiator transmits with one ormore combinations of analog beams (P^(i) ¹ _(A), . . . P^(i) ^(N) _(A))with N and beam indexes indicated within the MIMO setup frame. Each ofthe BRP packets is received by the responder with corresponding analogbeams (W^(i) ¹ _(A), . . . W^(i) ^(N) _(A)). For each combination (P^(i)^(j) _(A), W^(i) ^(j) _(A)) the effective channel is estimated, a metricis computed (e.g., MMSE, capacity) and a digital beamformer (P^(i) ^(j)_(BD)) is computed by the responder .e.g., by using the methodsdescribed above. Alternatively the channel information can be saved andthe beamformer computation can be performed only for the (P*_(A),W*_(A)) which achieves the best metric, e.g., the lowest MMSE or highestcapacity.

In a third step, the MIMO feedback frame (MF) contains the indizes ofthe best sector for the initiator (W*_(A)). Additionally, thecorresponding digital BF matrix P_(BD) is fed back in the MIMO feedback,according to the format specified within the MIMO Fbck Request andhaving a form as proposed within Table 3.

If feedback was requested but is not present within the feedback frame,then the initiator may send a renewed digital beamforming request in BRPframe with the best analog beam combination and training sequencesattached, which are sent with these best analog beams, thus to enablethe computation of the digital beamformer at the responder. This framecan be sent as standalone or aggregated to the MIMO feedback for theresponder link or simply training sequences can be sent attached to theMIMO feedback for the responder link. If feedback was requested but isnot present but the responder recommends to not use the digitalbeamformer for the particular channel realization, the initiator maychoose not to send a renewed request.

In a second exemplary embodiment illustrated in FIG. 5 the initiatorwants to train the analog and digital beamformers for the initiator toresponder link. In this example it is assumed that the decision makerfor the initiator link is the initiator and that the initiator relies onthe responder's receiver to compute and feedback the digital transmitbeamformers that the former will use for transmission to the responder.

Steps 1 and 2 are similar to the first example. However, since theinitiator is the decision maker for the initiator to responder link itrequires in the first MIMO feedback (MF1 block) the computed metrics forthe selected analog sectors indicated within the MIMO Setup, togetherwith the indexes for the corresponding sectors. The responder may signalif digital beamforming information is ready or a retransmission with thebest analog beams is required. Based on these the initiator computes thebest combination and feeds back to the responder during MF2.

To allow the responder to compute the beamformers TRN sequences may beappended to MF2 or BRP frame can be aggregated which has TRN sequencesattached. These allow the channel estimation at the responder and thecomputation of the digital beamformers for the analog link chosen by theinitiator (W_Â*, P_Â*) as described above. During MF3 the responderfeeds back the computed matrix or requests retransmission with theanalog beams.

The capabilities may be signaled in the following format:

B0 B4 Requested B5 B6 B7 B8 Digital Digital BRP SC MU-MIMO SU-MIMO GrantNoRSS TX BF RX BF Blocks Supported Supported Required SupportedSupported Type Reserved Bits: 5 1 1 1 1

Digital TX BF Supported may be set to 1 if it can compute the transmitBFs, if it can apply FIR or just constant coefficient multiplicationsand if FIR applied which is the supported length. Optionally, if thereceiver does not have the capability to compute the transmitbeamforming coefficients, then it should feedback to the transmitter thetype of equalizer used and the corresponding parameters. If transmittingthis information is undesired, a default equalizer can be defined andthis will be used in the derivation of the precoding matrices.

FIG. 6 shows a diagram illustrating the operations of digitalbeamforming training after RTS/CTS (request to send/clear to send) withacknowledgement. It shows particularly an example of a hybrid BF phaseto compute the digital beamforming information for the initiator link(in the direction from the initiator (sending) to the responder(receiving)) when the AWV (antenna weight vector) configuration isincluded in RTS/CTS and the responder chooses the TRN configuration fordigital beamforming training. In this case the analog beamformers (e.g.AWVs) for which the digital beamforming information is computed is setto the one indicated within the control trailer CT 601 of the RTS/CTS.For BF training BRP packets 602 a and 602 b are transmitted. The purposeof the BRP packet 602 b from the responder is to acknowledge that it isready to perform the computation of the digital beamforming informationand to optionally request a number of TRN units (N_(TRN)) to allow it toestimate the effective channel with a desired precision and based onwhich the digital beamforming information can be computed. In moredetail, after the initiator has received the BRP packet 602 b, ittransmits a BRP packet 603 to the responder which is appended by N_(TRN)TRN units. During reception of the BRP packet 603, the responderestimates the channel H_(eff) and computes feedback matrices for eachTRN unit. These matrices are sent back from the responder to theinitiator as part of the MIMO feedback frame 604.

FIG. 7 shows a diagram illustrating the operations of digitalbeamforming training after RTS/CTS without acknowledgement. In contrastto FIG. 6, BRP packets 602 a and 602 b are not transmitted. It showsparticularly an example of a hybrid BF phase when the AWV configurationis in RTS/CTS and number of TRN units N_(TRN) is defined in a previousstage, e.g. during RTS/ CTS, or is fixed to a standard value, e.g.depending on a number of taps resolution, a number of RF chains and/or anumber of transmit streams. In this case the RTS/CTS contains a preciseindication (e.g. included in control trailer CT 701) that the followingtransmission is for digital beamforming training. In more detail, afterRTS/CTS exchange, BRP packet 703 is transmitted from initiator toresponder which is appended by N_(TRN) TRN units. During reception ofthe BRP packet 703, the responder estimates the channel H_(eff) andcomputes feedback matrices for each TRN unit. These matrices are sentback from responder to initiator as part of the MIMO feedback frame 704.

FIG. 8 shows a diagram illustrating the operations of digitalbeamforming directly after analog beamforming training. It showsparticularly an example of a hybrid BF phase when the AWV configurationwhich was chosen based on the analog beamforming feedback information isincluded in the BRP frame 802 a. In this case, the BRP frame 802 aincludes a field within the BRP request stating the TX RX sectorcombination which has been chosen as part of the latest MIMO analogbeamforming training. Within the same BRP frame 802 a, the BRP requestadditionally contains the digital BF request, whereas the DMG refinementelement contains the digital BF feedback request. This allows startingthe digital beamforming training directly after the analog beamformingtraining without contending for the medium (and requiring RTS/CTSexchange 601 and 701). This requires a one bit indication in the MIMOFBCK 801 or a description within a standard exposing the hybrid BFprotocol, to specify the time interval after the MIMO FBCK 801 receptionthat the initiator sends one or more BRP requests 802 a for the digitalbeamforming.

In more detail, after MIMO FBK exchange 801 which is to identify the AWVconfiguration used for analog beamforming, the initiator sends a BRPpacket 802 a to the responder holding a request for digital beamformingtraining as in 602 a and includes additionally AWV index used forsubsequent (e.g. after BRP packet 802 a) frame exchange. The AWV indexadded in the BRP packet 802 a makes the RTS/CTS exchange with controltrailer (601 or 701) obsolete. After having received the BRP packet 802a, the responder transmits a BRP request 802 b similarly to 602 bindicating readiness to compute digital beamforming information andoptionally TRN parameters such as number of TRNs to be used in next BRPpacket 803, e.g. N_(TRN). After the initiator has received the BRPpacket 802 b, it transmits a BRP packet (803) to the responder which isappended by N_(TRN) TRN units. During reception of the BRP packet 803,the responder estimates the channel H_(eff) and computes feedbackmatrices for each TRN unit. These matrices are sent back from responderto initiator as part of the MIMO feedback frame 804.

The proposed communication devices are generally capable to performdigital beamforming, wherein the digital beamforming will be performedduring the MIMO beamforming training, additional to analog beamformingtraining. The devices are further able to indicate both, wherein theability to perform beamforming is preferably indicated in thecapabilities field whereas the performing of digital beamforming duringthe MIMO beamforming training is preferably indicated in the setup frameor in beamforming refinement frames sent during the analog training.

In an embodiment the beamforming information is computed at theresponder. However, other solutions can be devised in which theinitiator computes the beamforming information according to channelinformation which is fed back by the responder. Hence, a flexible way ofsignaling may be provided.

Further, in an embodiment a predetermined metric is used and values of apredetermined metric are computed. Hereby, a predetermined metric is ametric based on which the best combination of analog beams is chosen.The digital beamforming information and the digital beamformers are notnecessarily computed based on this metric.

The present disclosure presents a solution to a practical problemappearing in mm-wave single carrier frequency domain equalization (SCFDE). The proposed solutions are preferably useful in mm-wavecommunication and avoid the drawbacks of known solutions, particularlyregarding full CSI estimation.

Thus, the foregoing discussion discloses and describes merely exemplaryembodiments of the present disclosure. As will be understood by thoseskilled in the art, the present disclosure may be embodied in otherspecific forms without departing from the spirit or essentialcharacteristics thereof. Accordingly, the disclosure of the presentdisclosure is intended to be illustrative, but not limiting of the scopeof the disclosure, as well as other claims. The disclosure, includingany readily discernible variants of the teachings herein, defines, inpart, the scope of the foregoing claim terminology such that noinventive subject matter is dedicated to the public.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. A single element or other unit may fulfill the functions ofseveral items recited in the claims. The mere fact that certain measuresare recited in mutually different dependent claims does not indicatethat a combination of these measures cannot be used to advantage.

In so far as embodiments of the disclosure have been described as beingimplemented, at least in part, by software-controlled data processingapparatus, it will be appreciated that a non-transitory machine-readablemedium carrying such software, such as an optical disk, a magnetic disk,semiconductor memory or the like, is also considered to represent anembodiment of the present disclosure. Further, such a software may alsobe distributed in other forms, such as via the Internet or other wiredor wireless telecommunication systems.

The elements of the disclosed devices, apparatus and systems may beimplemented by corresponding hardware and/or software elements, forinstance appropriated circuits. A circuit is a structural assemblage ofelectronic components including conventional circuit elements,integrated circuits including application specific integrated circuits,standard integrated circuits, application specific standard products,and field programmable gate arrays. Further a circuit includes centralprocessing units, graphics processing units, and microprocessors whichare programmed or configured according to software code. A circuit doesnot include pure software, although a circuit includes theabove-described hardware executing software.

It follows a list of further embodiments of the disclosed subjectmatter:

1. A communication device for RF-based communication with anothercommunication device, said communication device comprising:

digital beamforming circuitry (42) configured to perform digitalbeamforming based on digital beamforming information to obtain RF datastreams, and

analog beamforming circuitry (441, 442) configured to perform analogbeamforming for the obtained RF data streams,

wherein said analog beamforming circuitry is configured to performanalog beamforming training with the other communication device enablingthe other communication device to compute the digital beamforminginformation corresponding to one or more combinations of analog beamsused in said analog beamforming training, and

wherein said digital beamforming circuitry is configured to receive thecomputed digital beamforming information and to use it for performingthe digital beamforming.

2. The communication device as defined in embodiment 1,

wherein said analog beamforming circuitry is configured to receivevalues of a predetermined metric computed by the other communicationdevice for combinations of analog beams used in said analog beamformingtraining, to select the one or more best combinations of analog beamsbased on the values of the predetermined metric and transmit combinationinformation indicating the selected one or more best combinations ofanalog beams and/or training sequences with a selected set of transmitanalog beams to the other communication device enabling the othercommunication device to compute the digital beamforming information forthe selected best combinations.

3. The communication device as defined in any preceding embodiment,

wherein said analog beamforming circuitry is configured to receivechannel information from the other communication device with respect tothe channel during said analog beamforming training and to computevalues of a predetermined metric for combinations of analog beams usedin said analog beamforming training based on the received channelinformation.

4. The communication device as defined in any preceding embodiment,

further comprising antenna circuitry (45) configured to transmit theobtained RF data streams using the analog beams formed by the analogbeamforming circuitry,

wherein said antenna circuitry is configured to transmit a (first)digital beamforming indicator indicating that digital beamforming can beperformed by the communication device and/or a (second) digitalbeamforming indicator indicating that digital beamforming should beperformed during and/or after analog (MIMO) beamforming training inaddition to analog beamforming and/or to receive a (third) digitalbeamforming indicator indicating that digital beamforming can beperformed by the other communication device and/or a (fourth) digitalbeamforming indicator indicating that digital beamforming should beperformed during analog (MIMO) beamforming training in addition toanalog beamforming.

5. The communication device as defined in any preceding embodiment,

further comprising antenna circuitry (45) configured to transmit theobtained RF data streams using the analog beams formed by the analogbeamforming circuitry,

wherein said antenna circuitry is configured to transmit a decisionindicator indicating if the computation of the digital beamforminginformation is to be made by the communication device or the othercommunication device and/or indicating if the selection of the one ormore best combinations of analog beams during the analog beamformingtraining to be used for computation of the digital beamforminginformation is to be made by the communication device or the othercommunication device.

6. The communication device as defined in embodiment 4 and/or 5,

further comprising antenna circuitry (45) configured to transmit theobtained RF data streams using the analog beams formed by the analogbeamforming circuitry,

wherein said antenna circuitry is configured to transmit said digitalbeamforming indicator, in particular said second digital beamformingindicator, and/or said decision indicator in a setup frame or in framesused during (or defined for) analog beamforming training.

7. The communication device as defined in any preceding embodiment,

wherein said digital beamforming circuitry is configured to receiveprecoding information as digital beamforming information and to use saidprecoding information for precoding the RF data streams.

8. The communication device as defined in embodiment 7,

further comprising antenna circuitry (45) configured to transmit theobtained RF data streams using the analog beams formed by the analogbeamforming circuitry,

wherein said antenna circuitry is configured to transmit a precodingindicator (also called digital beamforming control information orprecoding control information or precoding parameter information) to theother communication device, said precoding indicator indicating thenumber of taps and/or the number of bits for quantization and/or theformat of the precoding information.

9. The communication device as defined in any preceding embodiment,

further comprising antenna circuitry (45) configured to transmit theobtained RF data streams using the analog beams formed by the analogbeamforming circuitry,

wherein said antenna circuitry is configured to embed precodingindication in a preamble or header of a frame of a transmit signal ofthe obtained RF data stream if at least a part of the said frame istransmitted with a digital precoding matrix applied.

10. A communication device for RF-based communication with anothercommunication device, said communication device comprising:

analog beamforming circuitry (511, 512) configured to perform analogbeamforming training with the other communication device for determininganalog beams for use in receiving said RF data streams, and

digital beamforming computation circuitry (54) configured to computedigital beamforming information corresponding to one or morecombinations of analog beams used in said analog beamforming trainingand to transmit the computed digital beamforming information to theother communication device enabling the other communication device toperform digital beamforming based on the digital beamforminginformation.

11. The communication device as defined in embodiment 10,

wherein said digital beamforming computation circuitry is configured tocompute values of a predetermined metric for combinations of analogbeams used in said analog beamforming training, to select the one ormore best combinations of analog beams based on the values of thepredetermined metric and to compute the digital beamforming informationfor the selected one or more best combinations.

12. The communication device as defined in embodiment 10 or 11,

wherein said digital beamforming computation circuitry is configured todetermine channel information with respect to the channel during saidanalog beamforming training, to transmit said channel information to theother communication device enabling the other communication device tocompute values of a predetermined metric for combinations of analogbeams used in said analog beamforming training based on the channelinformation, to receive combination information indicating the selectedone or more best combinations of analog beams from the othercommunication device and to compute the digital beamforming informationfor the selected one or more best combinations.

13. The communication device as defined in any one of embodiment 10 to12,

-   -   further comprising antenna circuitry (50) configured to receive        RF data streams, wherein said antenna circuitry is configured to        transmit a digital beamforming indicator indicating that digital        beamforming can be performed by the communication device and/or        that digital beamforming should be performed during MIMO        beamforming training in addition to analog beamforming and/or to        receive a digital beamforming indicator indicating that digital        beamforming can be performed by the other communication device        and/or that digital beamforming should be performed during MIMO        beamforming training in addition to analog beamforming.

14. The communication device as defined in any one of embodiment 10 to13,

further comprising antenna circuitry (50) configured to receive RF datastreams, wherein said antenna circuitry is configured to transmit thecomputed digital beamforming information in a feedback frame or in aframe used during (or defined for) analog beam-forming training.

15. The communication device as defined in any one of embodiment 10 to14,

wherein said digital beamforming computation circuitry is configured tocompute precoding information as digital beamforming information and totransmit said precoding information to the other communication devicefor precoding the RF data streams.

16. The communication device as defined in embodiment 15,

wherein said digital beamforming computation circuitry is configured touse effective channel information in the frequency domain, in particularchannel information obtained by frequency domain transformation orestimation of the analog beamformed channel, to derive time-domainbeamforming coefficients as digital beamforming information, inparticular beamforming coefficients as part of the digital beamforminginformation.

17. The communication device as defined in embodiment 16,

wherein said digital beamforming computation circuitry is configured tocompute a precoding matrix as precoding information, in particularwherein the computed precoding matrix satisfies Karush-Kuhn-Tuckeroptimality conditions of a sum-rate optimization with power constraintsor the precoding indication corresponds to the closest matrix in pointof an Euclidean distance or chordal distance to a digital beamformingcomputed based on eigenvectors of the frequency domain channel matricesfor the available frequencies or the precoding matrix corresponds to aneigenvector of a channel covariance matrix computed over all availablefrequencies.

18. A communication method for RF-based communication with anothercommunication device, said communication method comprising:

performing digital beamforming based on digital beamforming informationto obtain RF data streams, and

performing analog beamforming for the obtained RF data streams,

wherein said analog beamforming includes performing analog beamformingtraining with the other communication device enabling the othercommunication device to compute the digital beamforming informationcorresponding to one or more combinations of analog beams used in saidanalog beamforming training, and

wherein said digital beamforming includes receiving the computed digitalbeamforming information and using it for performing the digitalbeamforming.

19. A communication method for RF-based communication with anothercommunication evice, said communication method comprising:

performing analog beamforming training with the other communicationdevice for determining analog beams for use in receiving said RF datastreams,

computing digital beamforming information corresponding to one or morecombinations of analog beams used in said analog beamforming training,and

transmitting the computed digital beamforming information to the othercommunication-device enabling the other communication device to performdigital beamforming based on the digital beamforming information.

20. A non-transitory computer-readable recording medium that storestherein a computer program product, which, when executed by a processor,causes the method according to embodiment 18 or 19 to be performed.

21. A computer program comprising program code means for causing acomputer to perform the steps of said method according to embodiment 18or 19 when said computer program is carried out on a computer.

22. The communication device as defined in any one of embodiments 1 to17,

wherein the digital beamforming information comprises one time-domaincombining matrix or a FIR filter, whose components are derived based onchannel information in frequency domain, for a chosen analog beamcombination.

23. The communication device as defined in any one of embodiments 1 to17,

further comprising antenna circuitry configured to embed information inthe preamble/header of a frame in the transmit signal if the currentframe is transmitted with a digital precoding matrix applied so that thereceiver can then adapt its demodulation accordingly and knows if aproposed precoding is applied.

24. The communication device as defined in any one of embodiments 1 to9,

further comprising antenna circuitry (45) configured to transmit theobtained RF data streams using the analog beams formed by the analogbeamforming circuitry,

wherein said antenna circuitry is configured to apply single carriermodulation for transmitting data and/or the other communication deviceis configured to apply single carrier frequency domain equalization forreceiving data.

25. The communication device as defined in any one of embodiments 1 to9,

further comprising antenna circuitry (45) configured to transmit theobtained RF data streams using the analog beams formed by the analogbeamforming circuitry,

wherein said antenna circuitry is configured to receive precodinginformation in the form of beamforming coefficients of each data streamsto each transmit antenna.

26. The communication device as defined in any one of embodiments 1 to9,

further comprising antenna circuitry (45) configured to transmit theobtained RF data streams using the analog beams formed by the analogbeamforming circuitry,

wherein said antenna circuitry is configured to receive precodinginformation in the form of beamforming coefficients of each data streamsto each transmit antenna for multiple taps and corresponding delayinformation between the taps.

27. A communication device for RF-based communication with anothercommunication device, said communication device comprising:

digital beamforming circuitry configured to perform digital beamformingbased on digital beamforming information to obtain RF data streams, and

analog beamforming circuitry configured to perform analog beamformingfor the obtained RF data streams,

wherein said analog beamforming circuitry is configured to performanalog beamforming training with the other communication device enablingthe communication device to compute the digital beamforming informationcorresponding to one or more combinations of analog beams used in saidanalog beamforming training, and

wherein said digital beamforming circuitry is configured to use thecomputed digital beamforming information for performing the digitalbeamforming.

28. The communication device as defined in any one of embodiments 1 to17,

wherein the digital beamforming information is computed and used as timedomain information, in particular for single carrier transmission.

1. A communication device for RF-based communication with anothercommunication device, said communication device comprising: digitalbeamforming circuitry configured to perform digital beamforming based ondigital beamforming information to obtain RF data streams, and analogbeamforming circuitry configured to perform analog beamforming for theobtained RF data streams, wherein said analog beamforming circuitry isconfigured to perform analog beamforming training with the othercommunication device enabling the other communication device to computethe digital beamforming information corresponding to one or morecombinations of analog beams used in said analog beamforming training,and wherein said digital beamforming circuitry is configured to receivethe computed digital beamforming information and to use it forperforming the digital beamforming.
 2. The communication device asclaimed in claim 1, wherein said analog beamforming circuitry isconfigured to receive values of a predetermined metric computed by theother communication device for combinations of analog beams used in saidanalog beamforming training, to select the one or more best combinationsof analog beams based on the values of the predetermined metric andtransmit combination information indicating the selected one or morebest combinations of analog beams and/or training sequences with aselected set of transmit analog beams to the other communication deviceenabling the other communication device to compute the digitalbeamforming information for the selected best combinations.
 3. Thecommunication device as claimed in claim 1, wherein said analogbeamforming circuitry is configured to receive channel information fromthe other communication device with respect to the channel during saidanalog beamforming training and to compute values of a predeterminedmetric for combinations of analog beams used in said analog beamformingtraining based on the received channel information.
 4. The communicationdevice as claimed in claim 1, further comprising antenna circuitryconfigured to transmit the obtained RF data streams using the analogbeams formed by the analog beamforming circuitry, wherein said antennacircuitry is configured to transmit a digital beamforming indicatorindicating that digital beamforming can be performed by thecommunication device and/or a digital beamforming indicator indicatingthat digital beamforming should be performed during and/or after analogbeamforming training in addition to analog beamforming and/or to receivea digital beamforming indicator indicating that digital beamforming canbe performed by the other communication device and/or a digitalbeamforming indicator indicating that digital beamforming should beperformed during analog beamforming training in addition to analogbeamforming.
 5. The communication device as claimed in claim 1, furthercomprising antenna circuitry configured to transmit the obtained RF datastreams using the analog beams formed by the analog beamformingcircuitry, wherein said antenna circuitry is configured to transmit adecision indicator indicating if the computation of the digitalbeamforming information is to be made by the communication device or theother communication device and/or indicating if the selection of the oneor more best combinations of analog beams during the analog beamformingtraining to be used for computation of the digital beamforminginformation is to be made by the communication device or the othercommunication device.
 6. The communication device as claimed in claim 4,further comprising antenna circuitry configured to transmit the obtainedRF data streams using the analog beams formed by the analog beamformingcircuitry, wherein said antenna circuitry is configured to transmit saiddigital beamforming indicator and/or said decision indicator in a setupframe or in frames used during analog beamforming training.
 7. Thecommunication device as claimed in claim 1, wherein said digitalbeamforming circuitry is configured to receive precoding information asdigital beamforming information and to use said precoding informationfor precoding the RF data streams.
 8. The communication device asclaimed in claim 7, further comprising antenna circuitry configured totransmit the obtained RF data streams using the analog beams formed bythe analog beamforming circuitry, wherein said antenna circuitry isconfigured to transmit a precoding indicator to the other communicationdevice, said precoding indicator indicating the number of taps and/orthe number of bits for quantization and/or the format of the precodinginformation.
 9. The communication device as claimed in claim 1, furthercomprising antenna circuitry configured to transmit the obtained RF datastreams using the analog beams formed by the analog beamformingcircuitry, wherein said antenna circuitry is configured to embedprecoding indication in a preamble or header of a frame of a transmitsignal of the obtained RF data stream if at least a part of the saidframe is transmitted with a digital precoding matrix applied.
 10. Acommunication device for RF-based communication with anothercommunication device, said communication device comprising: analogbeamforming circuitry configured to perform analog beamforming trainingwith the other communication device for determining analog beams for usein receiving said RF data streams, and digital beamforming computationcircuitry configured to compute digital beamforming informationcorresponding to one or more combinations of analog beams used in saidanalog beamforming training and to transmit the computed digitalbeamforming information to the other communication device enabling theother communication device to perform digital beamforming based on thedigital beamforming information.
 11. The communication device as claimedin claim 10, wherein said digital beamforming computation circuitry isconfigured to compute values of a predetermined metric for combinationsof analog beams used in said analog beamforming training, to select theone or more best combinations of analog beams based on the values of thepredetermined metric and to compute the digital beamforming informationfor the selected one or more best combinations.
 12. The communicationdevice as claimed in claim 10, wherein said digital beamformingcomputation circuitry is configured to determine channel informationwith respect to the channel during said analog beamforming training, totransmit said channel information to the other communication deviceenabling the other communication device to compute values of apredetermined metric for combinations of analog beams used in saidanalog beamforming training based on the channel information, to receivecombination information indicating the selected one or more bestcombinations of analog beams from the other communication device and tocompute the digital beamforming information for the selected one or morebest combinations.
 13. The communication device as claimed in claim 10,further comprising antenna circuitry configured to receive RF datastreams, wherein said antenna circuitry is configured to transmit adigital beamforming indicator indicating that digital beamforming can beperformed by the communication device and/or that digital beamformingshould be performed during MIMO beamforming training in addition toanalog beamforming and/or to receive a digital beamforming indicatorindicating that digital beamforming can be performed by the othercommunication device and/or that digital beamforming should be performedduring MIMO beamforming training in addition to analog beamforming. 14.The communication device as claimed in claim 10, further comprisingantenna circuitry configured to receive RF data streams, wherein saidantenna circuitry is configured to transmit the computed digitalbeamforming information in a feedback frame or in a frame used duringanalog beamforming training.
 15. The communication device as claimed inclaim 10, wherein said digital beamforming computation circuitry isconfigured to compute precoding information as digital beamforminginformation and to transmit said precoding information to the othercommunication device for precoding the RF data streams.
 16. Thecommunication device as claimed in claim 15, wherein said digitalbeamforming computation circuitry is configured to use effective channelinformation in the frequency domain to derive time-domain beamformingcoefficients as digital beamforming information.
 17. The communicationdevice as claimed in claim 16, wherein said digital beamformingcomputation circuitry is configured to compute a precoding matrix asprecoding information, in particular wherein the computed precodingmatrix satisfies Karush-Kuhn-Tucker optimality conditions of a sum-rateoptimization with power constraints or the precoding indicationcorresponds to the closest matrix in point of an Euclidean distance orchordal distance to a digital beamforming computed based on eigenvectorsof the frequency domain channel matrices for the available frequenciesor the precoding matrix corresponds to an eigenvector of a channelcovariance matrix computed over all available frequencies.
 18. Thecommunication device as claimed in claim 1, wherein the digitalbeamforming information is computed and used as time domain information.19. A communication method for RF-based communication with anothercommunication device, said communication method comprising: performingdigital beamforming based on digital beamforming information to obtainRF data streams, and performing analog beamforming for the obtained RFdata streams, wherein said analog beamforming includes performing analogbeamforming training with the other communication device enabling theother communication device to compute the digital beamforminginformation corresponding to one or more combinations of analog beamsused in said analog beamforming training, and wherein said digitalbeamforming includes receiving the computed digital beamforminginformation and using it for performing the digital beamforming.
 20. Acommunication method for RF-based communication with anothercommunication device, said communication method comprising: performinganalog beamforming training with the other communication device fordetermining analog beams for use in receiving said RF data streams,computing digital beamforming information corresponding to one or morecombinations of analog beams used in said analog beamforming training,and transmitting the computed digital beamforming information to theother communication device enabling the other communication device toperform digital beamforming based on the digital beamforminginformation.
 21. A non-transitory computer-readable recording mediumthat stores therein a computer program product, which, when executed bya processor, causes the method according to claim 19 be performed.
 22. Acommunication device for RF-based communication with anothercommunication device, said communication device comprising: digitalbeamforming circuitry configured to perform digital beamforming based ondigital beamforming information to obtain RF data streams, and analogbeamforming circuitry configured to perform analog beamforming for theobtained RF data streams, wherein said analog beamforming circuitry isconfigured to perform analog beamforming training with the othercommunication device enabling the communication device to compute thedigital beamforming information corresponding to one or morecombinations of analog beams used in said analog beamforming training,and wherein said digital beamforming circuitry is configured to use thecomputed digital beamforming information for performing the digitalbeamforming.